In recent years, an optical system for forming an image of an object in a space by using a reflective imaging element has been proposed (for example, Patent Documents 1 to 5). The optical system includes a reflective imaging element and an object, and an image to be displayed in a space is an image of the object, which forms at a position of planar symmetry with respect to the reflective imaging element as a plane of symmetry.
The reflective imaging element disclosed in Patent Document 1 has holes penetrating through a plate-like substrate along its thickness direction, such that an optical element composed of two orthogonally-disposed specular elements is formed on the inner walls of each hole (see FIG. 4 of Patent Document 1), or has a plurality of transparent cylindrical bodies protruding in the thickness direction of the substrate, such that an optical element composed of two orthogonally-disposed specular elements is formed on the inner wall surface of each cylindrical body (see FIG. 7 of Patent Document 1).
In the reflective imaging elements disclosed in Patent Documents 1, 2, and 5, tens to hundreds of thousands of square holes, each of whose sides measures about 50 μm to 1000 μm, are formed in a substrate having a thickness of 50 μm to 1000 μm, the inner surface of each hole being mirror coated by electroforming technique, nanoprinting technique, or sputtering technique.
An optical system in which a reflective imaging element is used utilizes specular reflection of the reflective imaging element, and, according to its principles, the ratio in size between the image of the object and the image appearing in the space is 1:1.
For reference sake, the entire disclosure of Patent Documents 1 to 5 is incorporated herein by reference.